Conventional two way radios operate in either duplex or simplex mode, the duplex mode is similar to a telephone system where the receive and transmit paths are both open and both parties can speak to each other with no other requirement.
The more common operation is simplex where the transmit path of each radio only works when the transmitter is keyed by the operation of a “press to talk switch” (PTT). The types of switch used vary and can be either part of a microphone as in the case of the hand held types in common use, or a switch box in a lead between the radio and a headset, as used in commercial and operational headsets.
Operators often have to operate a radio transmit switch while using their hands to do other things and certain systems incorporate voice activation where the radio is switched at the detection of the users voice from the microphone. This technique is not reliable with some applications and the need for a switch actuated by the user is still the only reliable means of controlling the radio.